While overloaded routers and Internet servers account for much of
today’s network congestion, the bottleneck within the "last mile" to
the customer’s office or home remains the most challenging to remedy.
The answer may lie in the new Local Multipoint Distribution Service
(LMDS), one of the least publicized yet perhaps most powerful emerging
communications technologies.
Not Yet in the Public Eye
Digital Broadcast Satellites, Switched Broadband, Hybrid Fiber/Coax,
and other forms of wireless cable have all received more attention than
LMDS. However, several technologies have matured in the last few years
that work to make LMDS even more feasible. Gallium Arsenide (GaAS)
integrated circuits, digital signal processors, video compression
techniques, and advanced modulation systems have all made significant
improvements in cost and performance. A major rewrite of
telecommunication regulations has removed many of the barriers
preventing companies from entering new businesses. The demand for
bandwidth, which was once limited to large corporations, governments,
and universities, is now rising at the consumer level thanks to the
Internet. These factors have combined to create a need for a technology
that has access to a huge amount of bandwidth and can be deployed for
low up-front costs.
Proponents of LMDS say the technology can easily satisfy these demands
and relieve bottlenecks by providing high-speed, highly reliable
connections from the workstation or LAN to the high-speed backbone. The
wireless delivery of LMDS, combined with the significant amount of
spectrum allocated for its use, promises to allow for the delivery of
high-quality telecommunications services such as multichannel video,
high-speed Internet service, and local telephone service at prices far
below those of incumbent providers.
LMDS is considered a proven technology, because it’s been tested by the
U.S. military and corporate pioneers such as SpeedUs.com, formerly
CellularVision USA, in New York. SpeedUs.com offers high-speed TV and
Internet access in New York City under a special commercial license.
Basically, LMDS is a wireless service transmitting fixed, broadband
microwave signals (actually millimeter-wave signals) in the 28 GHz band
of the spectrum within small cells roughly three miles in
diameter. LMDS’s gigantic appeal lies in its ability to offer a wide
range of one-way and two-way voice, video, and data service
transmission capabilities with a capacity many times larger than any
current wireless or non-wireless service.
Because of its multipurpose applications, LMDS has the potential to
become a major competitor to local exchange and cable television
services.
Greed for Speed
What currently passes as broadband speed pales to insignificance
when compared to the speeds promised by LMDS. Hewlett-Packard predicts
throughputs as fast as 1.5 gbps
downstream, with upstream rates as high as 200 mbps.
That’s enough bandwidth to transmit 8,000 high-density color
photographs per second, provide high-speed Internet access at 100 times
current modem rates, or carry over 200 video channels simultaneously.
The residents in most homes in a neighborhood will be able to watch
separate digital movies, teleconference, and surf the Internet at high
speed all at the same time.
LMDS will provide customers with telephone service, multichannel video
programming, video communications, and two-way data services. While
LMDS isn’t linked to any one technology, it is, however, a very large
data pipe.
According to Ihor Nakonecznyj, senior manager of product marketing at
Nortel (Northern Telecom) Broadband Wireless Access, LMDS differs from
the ordinary data transport systems in the way a train differs from a
pipeline. "Both are transport systems, but a pipeline can transport
only one product from one place to another. A train, on the other hand,
can transport many different products over the same infrastructure.
LMDS, implemented with a multi-service protocol such as asynchronous
transfer mode (ATM), can transport, among others, voice, Internet,
Ethernet, video, computer files, and transaction data."
It is the multipoint radio technology, combined with the appropriate
protocol, access method, and speed, that gives LMDS the potential to
transform society.
Among the overwhelming advantages of LMDS is reliability. “As a
transport system, LMDS can be engineered to provide 99.999 percent
availability, rivaling that of the best fiber backbones,” said
Nakonecznyj.
The "Negroponte Flip"
Just as television and radio are becoming wired, telephones and
computers are becoming wireless, a paradigm shift now called the
"Negroponte Flip," first articulated by Nicolas Negroponte, director of
the Massachusetts Institute of Technology’s eminent Media Lab.
In the past, communication technologies exploited the lower end of the
radio frequency spectrum because, when boosted with enough power,
low-frequency signals can be transmitted long distances and even
penetrate buildings, as is the case with television and radio signals.
LMDS, on the other hand, uses low-powered, high-frequency signals (the
Ka band lies above the UHF band and below the far infrared region) over
short distances.
Because of this short distance, LMDS systems are configured in
stationary, line-of-sight cells. These cells are typically spaced on a
three-mile radius, with a single hub transceiver in the center
communicating at Gigabits per second with special devices affixed to
residences and businesses in the cell. LMDS cell layout determines the
cost of building transmitters and the number of households covered.
Direct line-of-sight between the transmitter and receiver is a
necessity. Reflectors or repeaters can spray a strong signal into
shadow areas to allow for more coverage. Various isolation techniques
are used to prevent interference between signals.
Tests have determined that a single transmitter would reach only
slightly more than 60 percent of the homes in a cell. With overlapping
cells and repeaters, however, that number jumps to almost 85 percent of
the homes.
Cell size is also influenced by the amount of local rainfall. Because
LMDS signals are microwaves, they are attenuated by water and lose
strength. To correct this, LMDS operators can either increase the power
of their transmissions when it rains in an attempt to ensure a strong
signal that reaches its destination, or they can reduce their cell
size.
Leaves, trees, and branches can also cause signal loss, but overlapping
cells and roof-mounted antennas generally overcome that problem.
The sheer size of the LMDS spectrum and the expectation that it will be
lightly regulated are other attractive aspects of LMDS. And since LMDS
can be used for two-way transmission through the use of low-powered
residential transceivers, it is seen as a way to provide interactive
services without the installation and maintenance expense encountered
on fiber or coax lines.
Talking from Both Sides
LMDS talks out of both sides of its connection mouth simultaneously.
This concurrent, two-way, wireless microwave transmission of mixed
video, audio, and data is possible thanks to an invention by electrical
engineers at the Microwave Laboratory at the Illinois Institute of
Technology (IIT).
The IIT invention permits transmission of multiple mediums within one
microwave system, all handled simultaneously. The new IIT system uses
the same principle as radio, where there are many stations, containing
a spectrum of information, broadcasting at the same time.
There are numerous directions in which the new IIT technology can be
applied. Among the applications made possible are the simultaneous
transmission of video conferencing, movies on demand, home shopping,
high-definition television (with as many as 500 channels), common
carrier telephone service, and various satellite communication
applications.
"Advantages of this system include large capacity, fast deployment, and
low-cost maintenance," said principal researcher Thomas Wong, director
of IIT’s Microwave Laboratory. Each signal requires its own carrier
frequency. A device’s bandwidth is the range of carrier frequencies
within which it can operate. Wong’s invention allows a single device to
send and receive multiple signals with a higher carrier density than
conventional designs. It is particularly effective in and above the Ka
band (26.5 to 40 GHz) of the microwave spectrum used by LMDS.
Wong’s work resulted from an effort to develop equipment for LMDS. Wong
has been granted a patent (U.S. patent No. 5701591) on the design of a
unique system that uses circular and elliptical polarizations (as
opposed to vertical and horizontal polarizations) in the transmission
and reception of signals, which makes his multifunction communication
system particularly well suited for urban environments. Polarization is
the orientation of the electric field of a horizontally propagating
electromagnetic wave.
When It Rains, It Distorts
There has been concern whether wireless communication can maintain data
integrity during sudden changes in weather. The concern is with data
drop-out or distortion every time it rains too hard or there’s a crack
of thunder.
Interference in the LMDS millimeter-wave signals results from physical
objects, overlapping signals, and weather. Absorption of microwaves by
water molecules in raindrops accounts for most of the signal
attenuation in the open space between the transmitter and the receiver,
causing microwaves to lose signal strength, a phenomenon engineers call
"rain fade."
"It is certainly true that rain has an effect on millimeter-wave
propagation," Wong said. "To overcome this attenuation, one needs to
put in reserved power in the transmission system. This is already
practiced in deployed systems." For weather systems similar to Chicago,
for instance, a 1-Watt per carrier transmitted power can provide
360-degree coverage for a radius of 5 kilometers when the antennas and
receivers used provide reasonable performance.
Wong’s system uses a different twist on polarization diversity to
increase the capacity of a communications link. “In C-band satellites,
for example, the use of vertical and horizontal polarizations in signal
transmission can double information capacity,” Wong said. “At
millimeter-wave bands, however, rainfall depolarizes linearly polarized
[vertical and horizontal] signals, rendering the use of such
polarization diversity inefficient. My invention makes use of
circular/elliptical polarizations and receiver designs to eliminate the
depolarization of effects of the propagation medium.”
There are numerous avenues for further research activity in this new
technical area. “As far as the deployment of systems utilizing the
current design we have developed, manufacturing capacity needs to be
geared up,” said Wong. “Current practice is to build the head-end and
relay stations in the United States and [the] subscriber units off
shore.”
Wong developed his invention under a collaborative agreement between
IIT and Telecommunications Equipment Corp., Palantine, Ill., which is
now in the process of developing commercial communications devices that
incorporate Wong’s multifunction microwave technology.
Business Prospects
With 1.3 GHz of spectrum, LMDS can provide a pipeline for an enormous
amount of data. Homeowners currently pay about $30 per month for CATV,
but businesses regularly pay over $1,000 per month for a high-speed T1
(1.544 Mbps) line from phone companies. Using only the 850 MHz
unrestricted bandwidth, along with a modulation scheme such as
quadrature phased shift keying (QPSK), well over 100 T1 equivalent
lines can be provided in a cell without splitting cells into separate
sectors. Even at half the price charged by a phone company, 100 leased
T1 lines would generate $50,000 in revenue per month in a cell. By
using horizontal and vertical polarized sectors in a cell, LMDS
providers will be able to re-use bandwidth and multiply the number of
T1 equivalents available.
A typical commercial LMDS application can potentially provide a
staggering downlink throughput of 51.84 to 155.52 Mbps and a return
link of 1.544 Mbps (T1). This capacity translates into phenomenal
potential to provide the "full-service network" packages of integrated
voice, video, and high-speed data services. Actual service-carrying
capacity depends on how much spectrum is allocated to video versus
voice and data applications.
Assuming that one GHz of spectrum is available, an all-video system
could provide up to 288 channels of digital broadcast quality
television, plus on-demand video services. Fast data, Internet access,
PCS backhaul, local loop bypass, digital video, digital radio, work at
home, and telemedicine are all possible. In fact, they are all possible
within the same cell.
Hubs and cell sites must be established early on for LMDS, but once
they are completed, new costs are incurred only as additional customers
sign on. The largest fixed expense associated with building out LMDS
cells will probably be the cost of subscriber equipment, not the
transmission and infrastructure equipment itself. By contrast, when
installing wire-line networks, the majority of the costs are incurred
before the first paying customer is even turned on.
Doug Page writes about science and technology from Redondo Beach,
California.
